Problem with polyphony - noteheads don't merge

[Stefan Hackl]

Hello,

I am typesetting a classical guitar piece, with four voices in the same
staff. I tried different possibilities to include

 \override Staff.NoteCollision
         #'merge-differently-headed = ##t

But the noteheads won't merge (see picture "unmerged.png"). If I replace
the note in the third voice by an invisible break ("e" is replaced by
"s"), it works perfectly. (picture: "merged.png").

Does anyone have an idea how to fix the problem?

I attach the complete file to this Email.

Thanks a lot!

Stefan

\version "2.8.1"

\header {
        title = "Introduktion und Variationen über einem Thema von Mozart "
        composer = "Fernando Sor "
}

flat = \markup { \raise #0.4 \smaller \musicglyph #"accidentals--2" }
sharp = \markup { \raise #0.6 \smaller \musicglyph #"accidentals-2" }
textcodaysym = \markup { \hspace #1 \raise #1.1 \musicglyph #"scripts-coda"}

StaffAVoiceA = \relative c {
        \voiceOne 
        \clef "G_8"
        \key g \major
        \time 4/4 
         \override Staff.NoteCollision
         #'merge-differently-headed = ##t                  
        e'2 e4.. e16 | e2 r4 r8. b16 ~ |   % 3
        < b g' > 2< a c fis > 4< g b e > | < b e > 2dis!4 r |   % 5
        fis2 fis4.. fis16 | fis2 r |   % 7
        < b, b'-2 > \grace { a'16 } < b, b' > 4\grace { a'16 } < b, b' > 4| < 
e-0 g-3 > 2r |   % 9
        e e4 e | fis4.. g16 fis4 fis |   % 11
        gis!2 gis4 gis | a4.. a16 c8 r r4 |   % 13
        a2 \times 2/3 { b8 e, f!  }\override TupletNumber #'transparent = ##t 
\times 2/3 { fis! g gis!  } | a2 < e g > 4r8. g16 |   % 15
        < g c, >4 fis8 r16 fis fis4 e8 r16 e | \times 2/3 { dis!8 b b  } \times 
2/3 {\stemDown b' \stemUp b, b  } \times 2/3 {\stemDown b''\stemUp b,, b  } 
a'8. a16 |   % 17
        g4 \times 2/3 {\stemDown b8 \stemUp b, b  } \times 2/3 {\stemDown b'' 
\stemUp b,, b  } e8. e16 | \times 2/3 { dis!8 b b  } \times 2/3 { \stemDown b' 
\stemUp b, b  } \times 2/3 { \stemDown b'' \stemUp b,, b  } a' r16 fis |   % 19
        g4 \times 2/3 {\stemDown  b8 \stemUp b, b  } \times 2/3 {\stemDown  
b''\stemUp b,, b  } e8. e16 | < fis, dis'! > 8r s4 s e'8. e16 |   % 21
        < fis, dis'! > 2r4 e'8. e16 | dis!4 s  \override TupletNumber 
#'transparent = ##f \times 2/3 { r4 b8  } e8. e16 |   % 23
        < fis, dis'! > 4r8 r4 < dis! b' > r8 | < dis! b' > 2r4\fermata r8 
b''16. a32 
        \bar "|."
}
StaffAVoiceB = \relative c {
        \voiceTwo 
         \override Staff.NoteCollision
         #'merge-differently-headed = ##t                  
        < e, b' e g b > 2< e b' e g b > < e b' e g b > s   % 3
        r4 e'2. < fis a > r4   % 5
        < b, dis! b' > 2< b dis! b' > < b dis! b' > s   % 7
        s1 s   % 9
        g2 g4 g a2 a4 a   % 11
        b2 b4 b c2 a8 r r4   % 13
        a'2 < g e' > 4s fis2 e4 r   % 15
        a,2 ais! < b fis' b > 4s s \times 2/3 { fis''8 b, b  } \override 
TupletNumber #'transparent = ##t  % 17
        \times 2/3 { e b b  } s4 s \times 2/3 { g8 b b  } < b, fis' > 4s s 
\times 2/3 { fis''8 b, dis!  }   % 19
        \times 2/3 { e b b  } s4 s \times 2/3 { g8 b g  } b,4 s2.   % 21
        \override TupletNumber #'transparent = ##f \times 2/3 { r8 b b  } 
\override TupletNumber #'transparent = ##t \times 2/3 { c  ( b ) b  } \override 
TupletBracket #'transparent = ##t \times 2/3 { ais!  ( b ) b  } b4 \times 2/3 { 
r8 b b  } \times 2/3 { c  ( b ) b  } \times 2/3 { ais!  ( b ) b  } b4 ~   % 23
        b r8 r4 b r8 b2 r2 
}
StaffAVoiceC = \relative c {
        \voiceThree 
        \override Staff.NoteCollision
         #'merge-differently-headed = ##t                   
        s1 s   % 3
        s s   % 5
        s s   % 7
        s s   % 9
        \times 2/3 { g8 \stemDown < e' b' > < e b' >  } \times 2/3 { < e b' > < 
e b' > < e b' >  } \times 2/3 {\stemUp g, \stemDown < e' b' > < e b' >  
}\override TupletNumber #'transparent = ##t \times 2/3 { \stemUp g, \stemDown < 
e' b' > < e b' >  } \times 2/3 {\stemUp a, \stemDown < e' c' > < e c' >  } 
\times 2/3 { < e c' > < e c' > < e c' >  } \times 2/3 {\stemUp a,\stemDown < e' 
c' > < e c' >  } \times 2/3 { \stemUp a,\stemDown < e' c' > < e c' >  }   % 11
        \times 2/3 {\stemUp b\stemDown < e d' > < e d' >  } \times 2/3 { < e d' 
> < e d' > < e d' >  } \times 2/3 {\stemUp b\stemDown < e d' > < e d' >  } 
\times 2/3 {\stemUp b \stemDown < e d' > < e d' >  } \times 2/3 { \stemUp c 
\stemDown <c' e > < c e >  } \times 2/3 { < c e > < c e > < c e >  } s2   % 13
        \times 2/3 {\stemUp a8 dis! e  } \times 2/3 { fis e dis!  } s2 \times 
2/3 { fis,8 b c  } \times 2/3 { cis! d dis!  } \times 2/3 { e, g gis!  } 
\stemDown \times 2/3 { a ais! b  }   % 15
        \times 2/3 { \stemUp a,  \stemDown < a' c > < a c >  } \times 2/3 { < a 
c > < a c > < a c >  } \times 2/3 {\stemUp ais,! \stemDown < g' cis! > < g cis 
>  } \times 2/3 { < g cis > < g cis > < g cis >  } s1   % 17
        s 
}
StaffAVoiceD = \relative c {
        \voiceFour 
        \override Staff.NoteCollision
         #'merge-differently-headed = ##t                   
        s1 s   % 3
        s s   % 5
        s s   % 7
        s s   % 9
        s s   % 11
        s s   % 13
        s s   % 15
        s s   % 17
        s s   % 19
        s \times 2/3 { fis8 b b  }\override TupletNumber #'transparent = ##t 
\times 2/3 { c b b-.  } \times 2/3 { ais! b b-.  } \times 2/3 { g b g  }   % 21
        s4 s \times 2/3 { s b8  } \times 2/3 { g b g  } s4 s s \times 2/3 { g8 
b g  } 
}
StaffA = \simultaneous {
        \context Voice="StaffAVoiceA" \StaffAVoiceA
        \context Voice="StaffAVoiceB" \StaffAVoiceB
        \context Voice="StaffAVoiceC" \StaffAVoiceC
        \context Voice="StaffAVoiceD" \StaffAVoiceD
}
\score {
        \relative <<
                \context Staff = cStaffAA <<
                        \context Voice = cStaffAVoiceA \StaffAVoiceA
                        \context Voice = cStaffAVoiceB \StaffAVoiceB
                        \context Voice = cStaffAVoiceC \StaffAVoiceC
                        \context Voice = cStaffAVoiceD \StaffAVoiceD
                >>

                \set Score.skipBars = ##t
                \set Score.melismaBusyProperties = #'()
        >>
}
\paper {
        #(set-paper-size "a4")
}